1. Field of the Invention
The present invention relates to a sampling point detection circuit for detecting a sampling point in an eye diagram, a transmission system comprising the sampling point detection circuit, a pre-emphasis intensity adjustment method using the sampling point detection circuit, a logic analyzer comprising the sampling point detection circuit, and an evaluation method for evaluating transmission characteristics of a digital signal in a transmission path based on an eye diagram of an output signal from an output end of the transmission path when an evaluation signal is input to an input end of the transmission path.
2. Description of the Related Art
As set forth in “Jitter Analysis Techniques for High Data Rates—Application Note 1432”, Agilent Technologies, http://cp.literature.agilent.com/litweb/pdf/5988-8425EN.pdf, in a digital transmission path used for high speed serial transmission, signal degradation occurs due to jitter, noise or transmission loss. For this reason, in order to improve communication reliability, a circuit designer adjusts transmission waveforms so as to obtain an optimal sampling point and transmission waveforms at an output end of the transmission path.
FIG. 19 is a diagram illustrating an eye pattern (eye diagram). Typically, in a region satisfying a certain error rate (hereinafter referred to as the “effective region”) in an eye diagram (though this is also referred to as the “eye pattern”, the term “eye diagram” is used in this patent specification) that is obtained when a pseudo-random data signal is input to a transmission path, a point where error is least likely to occur is found by using any technique and this point is defined as the sampling point.
For example, as set forth in Japanese Unexamined Patent Publication No. 2003-018140, a technique in which an intersection of a voltage level at which a time axis has the largest dimension and a time value at which an amplitude axis has the largest dimension in an effective region in an eye diagram is determined as a sampling point has been proposed.
Further, for example, as set forth in Japanese Unexamined Patent Publication No. 2006-234821, a technique in which squares (or circles) that are centered on respective points and that fit inside an effective region in an eye diagram are drawn with respect to all points in the effective region and the center point of the square (or circle) having the largest area is determined as a sampling point has been proposed.
In addition, in a technique widely used in the conventional art, a center point in the time and amplitude axis directions of an eye opening in an eye diagram is simply determined as a sampling point. (For example, see Japanese Unexamined Patent Publication No. 2007-274139 and Japanese Unexamined Patent Publication No. 2008-252696.)
On the other hand, when an eye opening in an eye diagram is reduced due to transmission loss of a signal received through a transmission path, transmission waveforms are adjusted to maximize the eye opening in the eye diagram by amplifying the signal input to the transmission path using a pre-emphasis circuit. Conventionally, the transmission waveforms are adjusted to maximize the eye opening in the amplitude axis direction (voltage direction) of the received waveforms in the eye diagram.
In contrast to this, for example, as set forth in Japanese Unexamined Patent Publication No. 2007-274139, there is also proposed a technique in which an eye opening in the time axis direction in an eye diagram is observed to adjust transmission waveforms.
Further, for example, as set forth in Japanese Unexamined Patent Publication No. 2008-252696, a technique in which attenuation of a reflected signal from a receiving end is observed to adjust transmission waveforms has been proposed.
On the other hand, as described in “A basic knowledge of jitter and an overview of measurement techniques in high speed serial communication”, Agilent Technologies, there is proposed a technique for evaluating quality of a digital signal in a transmission path used for high speed serial transmission. Evaluation parameters of the transmission path in a frequency domain include an S parameter that is calculated from waveforms observed by a TDR (time domain reflectometry)/TDT (time domain transmission) measuring device or measured by a network analyzer. Further, evaluation parameters of the transmission path in a time domain include characteristic impedance obtained from step response waveforms, an eye diagram obtained when a pseudo-random data signal is input to the transmission path, a jitter amount, and a bit error rate (BER).
FIG. 20 is a diagram illustrating an evaluation system that is typically used in the conventional art. In a transmission evaluation system 200, a transmission path 5 to be evaluated is connected to a pulse pattern generator (PPG) 101 and a digital oscilloscope 41 or a bit error rate tester (BERT) 42 via coaxial cables 102. When a pseudo-random data signal that is an evaluation signal generated by pulse pattern generator 101 is input to an input end of transmission path 5, a state of an output signal from an output end of transmission path 5 can be observed as an eye diagram (eye pattern), for example, by using digital oscilloscope 41.
Quality of the transmission path can be evaluated by the degree of openness of the eye in the observed eye diagram. The eye diagram visually displayed on a screen of the digital oscilloscope has an advantage that an observer can intuitively recognize it. However, this eye diagram is not suitable for quantitative evaluation.
For this reason, as set forth in Japanese Unexamined Patent Publication No. 2002-247116, an “eye opening ratio” that quantitatively represents how widely the eye is opened in the eye diagram is determined by calculation and the quality of the transmission path is evaluated based on this eye opening ratio.
FIG. 21 is a diagram for describing calculation of an eye opening ratio that is performed in the conventional art. In this figure, X represents a threshold level. The eye opening ratio is defined by a value in the time axis direction (jitter) and a value in the amplitude axis direction (noise). As illustrated in FIG. 21, in an eye opening in an eye diagram at a specific bit error rate, it is assumed that, on the amplitude axis, a dimension between the maximum and minimum amplitude levels is A and a dimension between the minimum amplitude level higher than X and the maximum amplitude level lower than X is B. Further, it is assumed that, on the time axis, the maximum and minimum dimensions of the eye opening are C and D, respectively. At this time, the eye opening ratio in the time and amplitude axis directions is expressed as follows:
            Eye      ⁢                          ⁢      opening      ⁢                          ⁢      ratio      ⁢                          ⁢              (        time        )              =                  2        ×        D                    C        +        D                        Eye      ⁢                          ⁢      opening      ⁢                          ⁢      ratio      ⁢                          ⁢              (        amplitude        )              =                  2        ×        B                    A        +        B            
As set forth in Japanese Unexamined Patent Publication No. 06-237231, an apparatus for automating evaluation of quality of a transmission path so as to avoid visual observation using an eye mask that has been performed for this purpose has been proposed. Further, as set forth in Japanese Unexamined Patent Publication No. 01-154660, a circuit for simplifying calculation of an eye opening ratio in an eye diagram has also been proposed.
Actual eye diagrams often have eye openings whose shape is asymmetrical in the amplitude and time axis directions. In this case, in the conventional techniques, the optimal sampling point cannot always be detected.
FIG. 22 is a diagram of detecting of a sampling point when an eye opening in an eye diagram has an asymmetrical shape in the time axis direction. According to the invention as set forth in Japanese Unexamined Patent Publication No. 2003-018140, an intersection of a voltage level at which a time axis has the largest dimension and a time value at which an amplitude axis has the largest dimension in an effective region in an eye diagram is assumed to be an optimal sampling point. Consequently, the sampling point is detected at a point P1. Thus, according to the invention as set forth in Japanese Unexamined Patent Publication No. 2003-018140, a sampling point can be accurately detected in an eye diagram in which an eye opening has a symmetrical shape in the amplitude and time axis directions. However, this technique is not practical because actual eye diagrams often have eye openings whose shape is asymmetrical in the amplitude and time axis directions.
On the other hand, according to the invention as set forth in Japanese Unexamined Patent Publication No. 2006-234821, squares (or circles) that are centered on respective points and that fit inside an effective region are drawn with respect to all points in the effective region in an eye diagram and the center point of the square (or circle) having the largest area is determined as a sampling point. Consequently, a sampling point can be accurately detected at a point P2. However, according to the invention as set forth in Japanese Unexamined Patent Publication No. 2006-234821, the squares (or circles) that are centered on the respective points and that fit inside the effective region have to be obtained with respect to all the points in the effective region in the eye diagram. As a result, this technique requires enormous processing time.
According to the inventions as set forth in Japanese Unexamined Patent Publication No. 2007-274139 and Japanese Unexamined Patent Publication No. 2008-252696, a center point in the time and amplitude axis directions of an eye opening in an eye diagram is simply determined as a sampling point. Consequently, when the eye opening of the eye diagram has a symmetrical shape in the amplitude and time axis directions, the sampling point can be accurately detected. However, when the eye diagram deviates in the time axis direction, the sampling point cannot be accurately detected.
As long as the sampling point cannot be accurately detected, various information processing devices such as a pre-emphasis circuit and a logic analyzer that perform processes with respect to the sampling point cannot accurately operate.
Further, even if different eye diagrams have the same eye opening ratio values, the corresponding transmission paths may not have entirely comparable quality. FIGS. 23A and 23B are diagrams of the evaluation of quality of a transmission path when an eye diagram is gently opened. Though the eye diagrams illustrated in FIGS. 23A and 23B both have an eye opening ratio of 75% in the time axis direction and an eye opening ratio of 75% in the amplitude axis direction, the eye diagram illustrated in FIG. 23B has an area of the eye that is smaller than that illustrated in FIG. 23A. As a result, even though the diagram of both eyes has the same eye opening ratio, it should be understood that the eye diagram illustrated in FIG. 23B has lower quality than that illustrated in FIG. 23A. But, such evaluation cannot be made by using the conventional eye opening ratio values only. The qualitative evaluation like this can be made by visually observing the eye diagram. However, when there are many objects to be evaluated, the visual evaluation like this is time-consuming and inefficient.
Still further, conventionally, in evaluation of a transmission path, a value in the time axis direction that is affected by jitter and a value in the amplitude axis direction that is affected by noise as an eye opening ratio and the values in the time and amplitude axis directions are evaluated independently of each other. However, it is essentially desirable that quality of a digital signal passing through the transmission path is evaluated taking into consideration the effect of both the jitter and the noise.
Still further, in a mask test that tests an eye diagram by using a mask pattern, it can be evaluated only whether the eye diagram has good quality or not. However, the degree or specific details of the quality of the eye diagram cannot be quantitatively evaluated.
In view of the above problems, it is an object of the invention to provide a sampling point detection circuit that can accurately detect a sampling point in an eye diagram, a logic analyzer and a transmission system comprising this sampling point detection circuit, a pre-emphasis intensity adjustment method using this sampling point detection circuit, and an evaluation method for automatically evaluating, by a processor, transmission characteristics of a transmission path based on an eye diagram of an output signal from an output end of the transmission path when an evaluation signal is input to an input end of the transmission path.